One technique for printed wiring boards which are used in a variety of electronic devices is to provide decoupling capacitors between a power supply wiring layer and a ground wiring layer in order to reduce electromagnetic radiation caused by the power supply system (see Japanese Patent Laid-Open No. H11-54860). FIG. 3 and FIG. 4 are drawings for explaining an example of such a technique. FIG. 3 is a transparent plan view of a multilayer printed wiring board 1, and FIG. 4 is a cross-sectional view of a central portion of the multilayer printed wiring board 1 of FIG. 3. Note that, in both drawings, the same symbols are used to denote the same portions.
As shown in FIG. 4, the multilayer printed wiring board 1 has a multilayer structure including a first insulation layer 10, a ground wiring layer 2, a second insulation layer 11, a power supply wiring layer 3, and a third insulation layer 12 which form layers in the stated order. Thus, the power supply wiring layer 3 and the ground wiring layer 2 oppose each other via the second insulation layer 11.
Further, a signal wiring layer 13 is formed on the first insulation layer 10 and a signal wiring layer 14 is formed on the third insulation layer 12. The ground wiring layer 2 (portion shown with hatching sloping downwards from right to left in FIG. 3) and the power supply wiring layer 3 (portion shown with hatching sloping downwards from left to right in FIG. 3) are provided over nearly the entire area of printed wiring board 1 in an arrangement known as “solid” wiring (all-over printed wiring).
As shown in FIG. 3, integrated circuits 5 to 8 are mounted on the multilayer printed wiring board 1. In proximity to the integrated circuits 5 to 8, decoupling capacitors 9a to 9l for suppressing the noise of each circuit, are mounted. Further, a plurality of decoupling capacitors 110a to 110l are mounted at a perimeter region of the multilayer printed wiring board 1.
In order to supply power to the power supply wiring layer 3, a power supply connector 4 is mounted at a bottom left-hand edge portion of the multilayer printed wiring board 1. The power supply connector 4, as shown in detail in FIG. 5, includes a power supply through hole 41 and a ground through hole 42. The power supply through hole 41 is electrically connected to the power supply wiring layer 3 and the ground through hole 42 is electrically connected to the ground wiring layer 2.
Here, as can be seen in FIG. 6, each of the decoupling capacitors 9a to 9l (which are collectively denoted 9) mounted around the each of the integrated circuits 5 to 8 has one end connected to the power supply wiring layer 3 by power supply wiring 53 and the power supply through hole 51. The other end of each capacitor 9 is connected to the ground wiring layer 2 by ground wiring 54 and ground through hole 52.
Further, as can be seen from FIG. 7, each of the decoupling capacitors 110a to 110l (collectively denoted 110) mounted in proximity to the printed wiring board 1 has one end connected to the power supply wiring layer 3 by power supply wiring 163 and power supply through hole 161. The other end of each of the capacitors 110 is connected to the ground wiring layer 2 by ground wiring 164 and ground through hole 162.
The power supply wiring layer 3 is given a solid wiring form within the printed wiring board 1, so that the power supply through hole 41 of the power supply connector 4 which is to form a power supply source and the power supply through holes 51 and 161 which are to form power supply destinations, are all included in an area covered by the wiring layer 3. Similarly, the ground wiring layer 2 is given a solid wring form within the printed wiring board 1 so that the ground through hole 42 of the power supply connector 4 and the ground through holes 52 and 162 of the decoupling capacitors, are all included in an area covered by the wiring layer 2.
With this construction, when the integrated circuits 5 to 8 mounted on the multilayer printed wiring board 1 operate, power supply current flows in the power supply wiring layer 3 and the return path current of the power supply current flows in the ground wiring layer 2. Thus, as a result of the currents flowing in the wiring layers 2 and 3 which form parallel plates, resonance noise occurs between the power supply wiring layer 3 and the ground wiring layer 2.
Since the power supply wiring layer 3 includes the power supply through hole 41 of the power supply connector 4 which forms the power supply source and the power supply through holes 51 which form the power supply destinations and extends to a perimeter of the substrate, the resonance noise is transmitted to the perimeter of the power supply wiring layer 3, and appears as a noise voltage between the power supply wiring layer 3 and the ground wiring layer 2. As a consequence, problems such as EMI (Electro-Magnetic Interference) radiation and the like occur.
In order to absorb the noise voltage between the power supply wiring layer 3 and the ground wiring layer 2, the decoupling capacitors 110a to 110l are mounted in the perimeter portion of the printed wiring board 1. One end of each of the capacitors 110 is connected to the power supply wiring layer 3 and the other end is connected to the ground wiring layer 2.
As described above with reference to FIG. 3 and FIG. 4, in the multilayer printed wiring board, the power supply wiring layer 3 including all the power supply through holes (51 in FIG. 6 and 161 in FIG. 7) which form the power supply destinations is provided with a solid wiring configuration extending, within permitted range, over nearly the entire area of the multilayer printed wiring board 1. To provide a bypass for the noise voltage resulting from resonance between the power supply wiring layer 3 and the ground wiring layer 2, a large number of dedicated decoupling capacitors 110a to 110l must be provided in the peripheral region of the power supply wiring layer 3. This configuration therefore has the disadvantage of a large number of components.